1. Field of the Invention
The present invention relates to a magnetic head, especially a magnetic head applied to a medium for high density recording.
2. Description of the Prior Art
Ferrite is conventionally used as a material for a magnetic head core, because it has superior workability and resistance to wear, however, its saturated magnetic flux density Bs is 35-50 % lower than that of alloy materials. The saturation magnetization, Bs, of the material of the head core becomes a problem when ferrite is used as materials for a head core for a medium of high density high coercive force recording, and from that point of view, a sendust or an amorphous alloy are preferred as core materials for a medium for high density recording.
When such alloys are used as materials for a magnetic head core, the specific resistance of the materials are as low as 70-120 .mu..OMEGA. cm, so that the loss of eddy current is large and the magnetic characteristics of the magnetic head formed as a bulk shape is insufficient in the high frequency region. As a result, an amorphous alloy or a sendust for the core material are produced as ribbons by the melt quenching method. The ribbon is layered on both sides by a non-magnetic substrate material with superior resistance to wear. On the other hand, by high densification or narrowing of the tracks makes it difficult to work with such ribbon by machine and creates many problems in manufacturing. Accordingly, the bets method among the conventional methods for producing a magnetic head is to form a thin film core material on a non-magnetic substrate by a thin film manufacturing method such as vapor deposition or sputtering. Furthermore, by using such a thin film core material as a magnetic head with high frequency applications preventing the eddy current loss, laminate type core material mutually formed from the magnetic material and the insulating material, can easily be obtained.
However, the difference between the thermal expansion coefficient of the magnetic material and the non-magnetic substrate causes the of following problems. First, since the thermal expansion coefficient of the sendust alloy is as large as about 150.times.10.sup.-7 /.degree. C., and there is no appropriate substrate material having such large thermal expansion coefficient, the magnetic head consists of the combination of the sendust and the substrate material by the adhesive. Further, it is difficult to maintain the gap distance with high precision by using such a method, and such a combination brings about the secular deterioration. Bonding by glass is the most reliable method for maintaining the gap length and the reliability. However, when the thermal expansion coefficients of the magnetic material and the substrate material do not coincide, cracks occur and the magnetic head is thereby removed from the bonded surface. When the magnetic head material is made by each a thin film manufacturing method the magnetic film is stripped from the substrate when the thermal expansion coefficient of the magnetic material and the substrate material do not coincide.
Accordingly, a polycrystalline or single crystalline Mn-Zn ferrite material is used as an amorphous alloy substrate material,; because the thermal expansion coefficient coincides with that of the substrate material (the latter has thermal expansion coefficient is 110-120.times.10.sup.-7 /.degree. C.), and the workability and the resistance to wear are superior. These materials are, magnetic materials, and are not used near the gap portion of the magnetic head. Glass is used in maintaining the gap length in the above-mentioned prior art constitution (which are shown in Japanese Patent Unexamined publication Sho 58-133620, Sho 59-94219, etc.)
As shown in FIG. 1, the prior art arrangement has a magnetic material 3 sandwiched between layers of ferrite 1, and glass 2 is used to fill up the areas near the gap 4 in place of the ferrite 1. By such an arrangement, the magnetic head can produce an effective play back signal, however, as the ferrite exhibits deterioration in the S/N ratio from the alloy material, caused by the large degree of running noise, the magnetic head shown in FIG. 1 also has the same effects, although there is no ferrite present near the gap portion. Furthermore, a complex process is necessary for making such a magnetic head.
Another prior art arrangement of the magnetic head which excludes the ferrite on the tape running surface so as to make use of the characteristics of the metal materials is proposed (disclosed in Japanese Patent Unexamined Publication Sho 58-14313, etc.). As shown in FIG. 2, the tape running surface on which the gap 7 is formed, is composed of a metal magnetic material 6 and a non-magnetic substrate material 5. As a the non-magnetic substrate material 5, glass is generally used. The normal glass containing sodium or potassium can be used as the glass material, but is not suitable for use with the amorphous alloy, because the resistance to wear is inferior and the thermal expansion coefficient is low.
Recently, the use of photo-sensitive crystallized glass has been proposed. Therein, the components such as LiO or SiO.sub.2 are crystallized by exposure to light. Such materials are adopted as substrate materials because the materials have nearly the same thermal expansion coefficient as that of an amorphous alloy, and the resistance to wear is superior.
On the other hand, use of a ceramic substrate containing CaO-SrO-TiO.sub.2 (disclosed in Japanese Patent Unexamined Publication Sho 52-57218), the ceramic substrate containing NiMnO.sub.2 (disclosed in Japanese Patent Unexamined Publication Sho 53-16399) or the ceramic substrate containing MgO-TiO.sub.2 (disclosed in Japanese Patent Unexamined Publication Sho 58-139322) as the substrate material have been proposed. These substrate materials can have various thermal expansion coefficients selectable in wide range by controlling their composition.
The crystallized glass or the ceramic substrates containing CaO-SrO-TiO.sub.2 are, however, chemically unstable due to the presence of alkaline metals or Ca as components, so that the characteristics of the resultant magnetic head become inferior by the adherence magnetic powder on the substrate surface during the running of the magnetic tape or the like. The ceramic substrate containing NiMnO.sub.2 has the disadvantages of high manufacturing cost, poor grindability, and low working efficiency in working to a usable shape, because MnO easily changes to Mn.sub.2 O.sub.3 by oxidization. The magnetic powder from the magnetic tape does not adhere to the substrate surface.
On the other hand, although the above-mentioned problems are not encounted in using a ceramic substrate containing MgO-TiO.sub.2. When the thermal expansion coefficient of the ceramic substrate is over 95 .times.10.sup.-7 /.degree. C. (25.degree.-400.degree. C.) the mol ratio of MgO versus TiO.sub.2 exceeds 2:1, and the segregated MgO is contained in the sintered body, and causes undesirable deliquescence under high humidity atmosphere.
Considering the above-mentioned prior art together, the substrate material for the magnetic head is required to have the following characteristics.
(1) Having a thermal expansion coefficient equal or near to that of the magnetic metal material. PA1 (2) Having superior machine workability. PA1 (3) Having superior resistance to wear. PA1 (4) Having a non-magnetized character. PA1 (5) On-adherance of magnetic powder from the magnetic tape. PA1 (6) Being chemically stable. PA1 (7) Being thermally stable, in order not to be damaged by heat of bonding glass. PA1 a magnetic core formed of a soft magnetic material, and PA1 a substrate for supporting the magnetic core, mainly composed of .alpha.-Fe.sub.2 O.sub.3 or a sintered substrate composed of 57-96 mol % of MgO, 2-41 mol % of NiO, and 2-41 mol % of TiO.sub.2.